Large electronic systems rely on cables to carry electrical power and signals between units and locations housing such electronics units. The single most important element in the maintenance of system protection from electromagnetic pulse effects and various noise sources is the integrity of the shielding of signal and power cables (coaxial and multiconductor) and any metallic tubing that provides such shielding. These system elements are prone to degradation over time from various environmental sources such as, for example, corrosion, oxidation, and mechanical stress. Such degradation effects may lead to loss of protection, compromise of security, or to intermittent equipment malfunction. Previous inspection methods and apparatus required partial disassembly of the cables. Conduits to be inspected had to be disconnected for testing, and the analysis of the test results required trained and highly skilled technical personnel for accurate detection and diagnosis.
Previous inductance/resistance test sets did not allow cables and conduits to be inspected in-situ without disconnection. The present invention can be used by relatively unskilled personnel, and without disconnecting the cables. Environments where the cable shield fault locator would be used include fixed and mobile communications facilities, military and commercial aircraft, naval; ships, and combat vehicles (i.e., tanks).
Flaws in the shields of cables can often be traced to connector connections. U.S. Pat. No. 5,189,375 includes a method and apparatus which can be used to inspect for resistive joints that may occur in ground/grounded connectors on cables, including shielded cables. Resistive joints are not the only flaws in cable shielding integrity, however. These flaws can result from an improper bond between cable shield and connector back shell, mechanical stress, metallic oxides, and the like, which introduce resistances in series with the shield and reduce the effectiveness of the shielding by introducing electromagnetic flux disturbances in the shielding path. When resistive flaws at connectors were found to be present, they could sometimes be detected by direct measurement of the cable shield resistance with, for example, a milliohmmeter.
Such shielding integrity flaws as occur at points along the length of the cables, however, have been more difficult to detect.
U.S. Pat. No. 5,391,991, also shows a resistive shielding flaw detector. The U.S. Pat. No. 5,189,375 also shows a method and apparatus for locating shielding integrity flaws by resistance techniques. The presence of a flaw in the shield will be indicated by an increase in the transfer impedance (ohms per unit length) or shield resistance (ohms) above a precisely established maximum permissible value. Typically acceptable values of cable shield resistance in working systems normally range from about ten milliohms (0.01 ohm) to several tenths of an ohm, depending upon cable parameters as length, diameter, characteristics of the shield material, and allowable junction resistances.
It is not always desirable to attempt detection of cable shield flaws by measurements of transfer impedance or shield resistance by standard techniques. The techniques described in U.S. Pat. No. 5,189,375 require that the equipment terminating the cable under test be disconnected for the resistance test. When measurements are made on a cable disconnected from its terminating equipment, a serious flaw may go undetected. Disconnection may relieve the mechanical stress that caused the defect, or may eliminate a resistive junction between a cable connector and the equipment connector. The movement may also create an additional defect that may easily be traced and corrected, leaving the original problem uncorrected. Additional serious degradation in cable shield protection may or may not be detectable with these techniques. The defects may reside in the cable shielding per se rather than in the connectors. Shield defects may exist between the equipment connector and the equipment itself. Thus, inspection for cable shield flaws should be performed with all the cables installed so that all sources of shield degradation will be present and detectable in the normal operating environment.
Other flaws may result in serious electromagnetic flux disturbances passing through the cable shield. These flaws are often very difficult to detect and locate without disassembly of the cables from the equipment, and may not be easily detected. Additional flaws may be caused by reinstallation of the cables. Therefore, detection of shield flaws caused by shield degradation should be performed with the cables normally connected.